Twitter just made a tweak to its iOS, Android, and web apps that allows previews of shared video and photos in the tweet stream itself. This might seem like a small change, but of course, it's part of a much bigger strategy as Twitter prepares for its IPO.
Previously, viewing linked photos or Vine videos (media shared via Twitter's official channels) in a tweet would require a click-through. Now a small image will be shown in the tweets themselves to give you a sneak peek. It's no doubt Twitter will use this system to show ads in tweets, too—what Twitter user would click on a link they knew would take them to an ad??
Simultaneously with this change, Twitter is making it easier for users to favorite, retweet, and reply to tweets directly from within the timeline—again trying to drive up user engagement. This is another move to consolidate Twitter's systems in the run-up to its IPO.